The present invention generally relates to an improved ignition system for use in an internal combustion engine. More particularly, the present invention relates to a capacitive discharge ignition system which provides improved spark energy and duration when compared with systems of the prior art.
Breakerless ignition systems for small gasoline engines have generally been divided into two broad classes, i.e., inductive type and capacitive discharge ("CD") type. Each of these types includes a transformer having a primary coil and a secondary coil wound about a magnetically permeable core. A magnet assembly is provided to revolve about an axis in synchronism with operation of the engine such that its pole faces are periodically moved past opposing pole faces of the core. As a result, voltages are induced in the transformer coils.
The inductive ignitions generally include a transistor, typically a darlington transistor, connected in circuit with the primary coil. The transistor is switched "on" to provide a low impedance path for current produced by the induced voltage, then switched off to interrupt the current. The interruption of the current causes a desired higher voltage to be induced on the secondary coil, which is connected to the engine's spark plug.
While inductive ignitions are often characterized by a high energy spark of relatively long duration, CD ignitions are often preferred for economic or other considerations. Like an inductive ignition, CD ignitions also include a step-up transformer mounted to cooperate with a revolving magnet assembly. These ignitions, however, further include a charge coil connected to a capacitor. Revolution of the magnet assembly results in a charge being accumulated on the capacitor.
An electronic switch, typically a silicon controlled rectifier ("SCR"), is connected between the capacitor and the primary coil. When the electronic switch is "closed," the charge which has accumulated on the capacitor produces a flow of current through the primary coil. As a result, a higher voltage is induced across the secondary coil to produce a spark at the spark plug. A typical example of a CD ignition system is shown in U.S. Pat. No. Re. 31,837, issued to Burson and incorporated herein by reference.
To initiate a spark across the gap of a spark plug, it is necessary for the voltage to first exceed a characteristic "spark ionization potential." After the spark has been initiated, it may be maintained by a characteristic "sustaining potential," which is generally much lower than the spark ionization potential. For example, a typical spark ionization potential may have a magnitude of ten (10) kilovolts or higher, whereas sustaining potentials of 300 to 700 volts are not uncommon.
In a CD ignition, the secondary coil voltage produced by discharge of the capacitor will, by design, exceed the required spark ionization potential. Typically, however, the capacitor will discharge relatively quickly. Thus, a secondary coil voltage produced solely by capacitive discharge will be correspondingly short in duration. For example, it is not uncommon for such a "discharge voltage" to have a duration of about 200 microseconds or less.
The prior art has recognized that the voltage induced on the secondary coil by revolution of the magnet assembly may exceed the sustaining potential at certain times during the revolution cycle if certain conditions are satisfied. As a result, triggering of the electronic switch at such times may produce a spark having a longer duration than that which may be produced by capacitive discharge alone. In this case, the capacitive discharge may be utilized to initiate the spark which can then be continued by the voltage induced by revolution of the magnet. This technique, which is similar to techniques often utilized in inductive ignitions, is described for a CD ignition in U.S. Pat. No. 4,538,586, issued to Miller ("the '586 patent").
A problem with utilizing the voltage induced on the secondary coil to sustain the spark in a CD ignition, such as that described in the '586 patent, has been the engine speeds required to produce the sustaining potential. Specifically, sufficient voltage has generally been induced on the secondary coil only when the engine is operating at relatively high speeds. At lower speeds, the spark duration has remained limited to that produced by discharge of the capacitor. Thus, inductive ignitions have often been utilized when longer spark duration has been desired.